Solar cells may be classified into inorganic solar cells and organic solar cells, depending on the thin-film material thereof. An organic solar cell is a device for directly converting solar energy into electrical energy using a photovoltaic effect.
A typical solar cell is manufactured based on a p-n junction by doping an inorganic semiconductor, namely crystalline silicon (Si). Electrons and holes, produced by absorbing light, are diffused to p-n junctions and accelerated by the electric field thereof and thus moved to the electrodes. In this procedure, the power conversion efficiency is defined as the ratio of the power applied to an external circuit to the solar power incident on a solar cell, and currently amounts to about 24% upon measurement under standardized virtual solar irradiation conditions. However, a conventional inorganic solar cell is limited in terms of profitability and the demand and supply of material, and thus, an organic semiconductor solar cell, the processing of which is easy and which is inexpensive and has various functionalities, is receiving attention as a long-term alternative energy source.
Solar cells may be classified into inorganic solar cells and organic solar cells, depending on the thin-film material thereof. In particular, an organic solar cell is a device for directly converting solar energy into electrical energy through a photovoltaic effect. Typically, an organic solar cell is configured such that a transparent substrate, a transparent electrode (an anode) formed of a tin-doped indium oxide thin film, a photoactive layer (a light absorption layer) and an aluminum electrode (a cathode) are sequentially stacked. The photoactive layer has a bulk heterojunction structure in which an electron acceptor and a hole acceptor or an electron donor are mixed. Since such an organic solar cell is advantageous in terms of low price, large area, and easy processing, the use thereof in flexible devices in the future is favorably expected, compared to low-molecule vacuum deposition type devices.
Recently, in order to effectively manufacture an organic solar cell using a printing process that is relatively inexpensive and enables high-speed production, a coating process for applying a transparent electrode such as ITO on a flexible substrate is regarded as very important.